<article id="wikiArticle">
<div></div>
<p><code><strong>indexOf()</strong></code> 方法返回调用  <a href="Reference/String" title="此页面仍未被本地化, 期待您的翻译!"><code>String</code></a> 对象中第一次出现的指定值的索引，开始在 fromIndex进行搜索。</p>
<p>如果未找到该值，则返回-1。</p>
<h2 id="Syntax" name="Syntax">语法</h2>
<pre><code class="language-javascript"><code><em>str</em>.indexOf(<em>searchValue</em>[, <em>fromIndex</em>]</code>)</code></pre>
<h3 id="Parameters" name="Parameters">参数</h3>
<dl>
<dt><code>searchValue</code></dt>
<dd>一个字符串表示被查找的值。</dd>
<dt><code>fromIndex </code><span class="inlineIndicator optional optionalInline">可选</span></dt>
<dd>表示调用该方法的字符串中开始查找的位置。可以是任意整数。默认值为 0。如果 <code>fromIndex &lt; 0</code> 则查找整个字符串（如同传进了 0）。如果 <code>fromIndex &gt;= str.length</code>，则该方法返回 -1。当被查找的字符串是一个空字符串，<code>fromIndex &lt;= 0</code>时返回<code>0</code>，<code>0 &lt; fromIndex &lt;= str.length</code>时返回<code>fromIndex</code>，<code>fromIndex &gt; str.length</code>时返回<code>str.length</code>。</dd>
<dt>
<h3 id="返回值">返回值</h3>
<p>指定值的第一次出现的索引; 如果没有找到 -1。</p>
</dt>
</dl>
<h2 id="Description" name="Description">描述</h2>
<p>字符串中的字符被从左向右索引。首字符的索引（index）为 0，字符串 <code>stringName</code> 的最后一个字符的索引是 <code>stringName.length - 1</code>。</p>
<pre><code  class="language-javascript">"Blue Whale".indexOf("Blue");     // returns  0
"Blue Whale".indexOf("Blute");    // returns -1
"Blue Whale".indexOf("Whale", 0); // returns  5
"Blue Whale".indexOf("Whale", 5); // returns  5
"Blue Whale".indexOf("", 9);      // returns  9
"Blue Whale".indexOf("", 10);     // returns 10
"Blue Whale".indexOf("", 11);     // returns 10</code></pre>
<h3 id="Example:_indexOf_and_case-sensitivity" name="Example:_indexOf_and_case-sensitivity">区分大小写</h3>
<p><code>indexOf</code> 方法区分大小写。例如，下面的表达式返回 -1：</p>
<pre><code  class="language-javascript">"Blue Whale".indexOf("blue") // returns -1
</code></pre>
<h3 id="检测是否存在某字符串">检测是否存在某字符串</h3>
<p>当检测某个字符串是否存在于另一个字符串中时，可使用下面的方法：</p>
<pre><code  class="language-javascript">"Blue Whale".indexOf("Blue") !== -1; // true
"Blue Whale".indexOf("Bloe") !== -1; // false</code></pre>
<h2 id="Examples" name="Examples">示例</h2>
<h3 id="Example:_Using_indexOf_and_lastIndexOf" name="Example:_Using_indexOf_and_lastIndexOf">使用<code>indexOf()</code> 和 <code>lastIndexOf()</code></h3>
<p>下例使用 <code>indexOf()</code> 和 <code>lastIndexOf()</code> 方法定位字符串中 "<code>Brave new world</code>" 的值。</p>
<pre><code  class="language-javascript">var anyString = "Brave new world";

console.log("The index of the first w from the beginning is " + anyString.indexOf("w"));
// logs 8
console.log("The index of the first w from the end is " + anyString.lastIndexOf("w")); 
// logs 10

console.log("The index of 'new' from the beginning is " + anyString.indexOf("new"));   
// logs 6
console.log("The index of 'new' from the end is " + anyString.lastIndexOf("new"));
// logs 6
</code></pre>
<h3 id="Example:_indexOf_and_case-sensitivity" name="Example:_indexOf_and_case-sensitivity"><code>indexOf</code> 和区分大小写</h3>
<p>下例定义了两个字符串变量。两个变量包含相同的字符串，除了第二个字符串中的某些字符为大写。第一个 <code>log</code> 方法输出 19。但是由于 <code>indexOf</code> 方法区分大小写，因此不会在 <code>myCapString</code> 中发现字符串<code> “cheddar"</code>，结果第二个 <code>log</code> 方法输出 -1。</p>
<pre><code  class="language-javascript">var myString    = "brie, pepper jack, cheddar";
var myCapString = "Brie, Pepper Jack, Cheddar";

console.log('myString.indexOf("cheddar") is ' + myString.indexOf("cheddar"));    
// logs 19
console.log('myCapString.indexOf("cheddar") is ' + myCapString.indexOf("cheddar")); 
// logs -1</code></pre>
<h3 id="Example:_Using_indexOf_to_count_occurrences_of_a_letter_in_a_string" name="Example:_Using_indexOf_to_count_occurrences_of_a_letter_in_a_string">使用 <code>indexOf</code> 统计一个字符串中某个字母出现的次数</h3>
<p>在下例中，设置了 <code>count</code> 来记录字母 <font face="Consolas, Liberation Mono, Courier, monospace">e</font> 在字符串 <code>str</code> 中出现的次数：</p>
<pre><code  class="language-javascript">var str = 'To be, or not to be, that is the question.';
var count = 0;
var pos = str.indexOf('e');

while (pos !== -1) {
  count++;
  pos = str.indexOf('e', pos + 1);
}

console.log(count); // displays 4
</code></pre>
<h2 id="规范">规范</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">规范</th>
<th scope="col">状态</th>
<th scope="col">说明</th>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/publications/files/ECMA-ST-ARCH/ECMA-262,%201st%20edition,%20June%201997.pdf" hreflang="en" lang="en" rel="noopener" title="ECMAScript 1st Edition (ECMA-262)">ECMAScript 1st Edition (ECMA-262)</a></td>
<td><span class="spec-Standard">Standard</span></td>
<td>初始定义。</td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/5.1/#sec-15.5.4.7" hreflang="en" lang="en" rel="noopener">ECMAScript 5.1 (ECMA-262)<br/><small lang="zh-CN">String.prototype.indexOf</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
<tr>
<td><a class="external" href="https://www.ecma-international.org/ecma-262/6.0/#sec-string.prototype.indexof" hreflang="en" lang="en" rel="noopener">ECMAScript 2015 (6th Edition, ECMA-262)<br/><small lang="zh-CN">String.prototype.indexOf</small></a></td>
<td><span class="spec-Standard">Standard</span></td>
<td> </td>
</tr>
</tbody>
</table>
<h2 id="浏览器兼容性">浏览器兼容性</h2>
<p></p><div class="blockIndicator warning"><strong><a class="external" href="https://github.com/mdn/browser-compat-data" rel="noopener">We're converting our compatibility data into a machine-readable JSON format</a></strong>.
            This compatibility table still uses the old format,
            because we haven't yet converted the data it contains.
            <strong><a class="new" href="/zh-CN/docs/MDN/Contribute/Structures/Compatibility_tables" rel="nofollow">Find out how you can help!</a></strong></div>
<div class="htab">
<a id="AutoCompatibilityTable" name="AutoCompatibilityTable"></a>
<ul>
<li class="selected"><a>Desktop</a></li>
<li><a>Mobile</a></li>
</ul>
</div><p></p>
<div id="compat-desktop">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Chrome</th>
<th>Firefox (Gecko)</th>
<th>Internet Explorer</th>
<th>Opera</th>
<th>Safari</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<div id="compat-mobile">
<table class="compat-table">
<tbody>
<tr>
<th>Feature</th>
<th>Android</th>
<th>Chrome for Android</th>
<th>Firefox Mobile (Gecko)</th>
<th>IE Mobile</th>
<th>Opera Mobile</th>
<th>Safari Mobile</th>
</tr>
<tr>
<td>Basic support</td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
<td><span style="color: #888;" title="Please update this with the earliest version of support.">(Yes)</span></td>
</tr>
</tbody>
</table>
</div>
<h2 id="See_also" name="See_also">相关链接</h2>
<ul>
<li><a href="Reference/Global_Objects/String/charAt" title="charAt() 方法从一个字符串中返回指定的字符。"><code>String.prototype.charAt()</code></a></li>
<li><a href="Reference/Global_Objects/String/lastIndexOf" title="lastIndexOf() 方法返回指定值在调用该方法的字符串中最后出现的位置，如果没找到则返回 -1。从该字符串的后面向前查找，从 fromIndex 处开始。"><code>String.prototype.lastIndexOf()</code></a></li>
<li><a href="Reference/Global_Objects/String/split" title="split() 方法使用指定的分隔符字符串将一个String对象分割成字符串数组，以将字符串分隔为子字符串，以确定每个拆分的位置。"><code>String.prototype.split()</code></a></li>
<li><a href="Reference/Global_Objects/Array/indexOf" title="indexOf()方法返回在数组中可以找到一个给定元素的第一个索引，如果不存在，则返回-1。"><code>Array.prototype.indexOf()</code></a></li>
</ul>
</article>